Polyesters such as polyalkylene terephthalates are widely usable in various resin industries due to their excellent physical and chemical properties. Especially, the polyester is highly useful for producing synthetic filaments or fibers which are also useful in various fields. However, since the polyester per se is highly hydrophobic, the polyester filaments are also hydrophobic and not at all suitable for use as filaments exhibiting a water and moisture absorbing property.
In order to obtain polyester filaments exhibiting a hydrophilic property, attempts were made to modify the known polyester filaments by producing them from a blend of a polyester with a polyalkylene glycol (U.S. Pat. No. 3,329,557 and British Pat. No. 956,833) or from a mixture of a polyalkylene glycol with an organic sulfonic metal salt (U.S. Pat. No. 3,682,846). However, the hydrophilic property of such resultant polyester filaments was found to be not only unsatisfactory but also readily degraded when the polyester filaments were laundered. In addition, the above-mentioned modification was found to cause the resultant polyester filaments to exhibit decreased physical properties, especially decreased resistance to actinic rays and a decreased thermal resistance.
In another attempt to obtain polyester filaments exhibiting a hydrophilic property, polyester filaments containing polyalkylene glycol or a mixture of polyalkylene glycol and organic sulfonic metal salt were treated with an alkali aqueous solution. This treatment resulted in formation of a number of fine concaves (long grooves) in the peripheral surface of the individual filament, the concaves extending approximately in parallel to the longitudinal axis of the filament and being effective for enhancing the water and moisture absorbing property of the filament. However, the resultant treated filament exhibited an extremely poor tensile strength, so that the filament could not be practically used.
In a recent attempt to obtain polyester filaments exhibiting a hydrophilic property, which was carried out by some of the inventors of the present invention, a hollow polyester filament containing an organic sulfonic metal salt which was not reactive to the polyester was treated with an alkali aqueous solution, so as to remove at least a portion of the organic sulfonic metal salt. This treatment resulted in formation of caves through which the hollow was connected to the outside of the filament. The resultant hollow filament had a satisfactory water and moisture absorbing property, and tensile strength. However, it was found that this type of the hollow filament exhibited a poor resistance to fibrillation when the filament was rubbed. This is because the caves were composed of long outside concaves formed in the peripheral surface of the filament, long pores formed the body of the filament and long inside concaves formed in the hollow surface of the filament, and; the outside and inside concaves and the pores extended in parallel to the longitudinal axis of the filament, and had a longitudinal size of 200 times or more the lateral size thereof. The long outside and inside concaves and the long pores promoted the fibrillation of the filament.
Under the above-mentioned circumstances, it is strongly desired to provide a polyester filament which has not only an excellent water and moisture absorbing property, but also, a satisfactory resistance to fibrillation of the filament.